What is a Phase Change?
Matter exists in three common states: solid, liquid, and gas. When energy is added to or removed from a substance, it can change from one state to another. These transitions are called phase changes (or changes of state).
A phase change is a transition of matter from one physical state (solid, liquid, or gas) to another, caused by the addition or removal of thermal energy.
The six main phase changes are:
- Melting — solid → liquid (energy absorbed)
- Freezing — liquid → solid (energy released)
- Vaporisation / Boiling — liquid → gas (energy absorbed)
- Condensation — gas → liquid (energy released)
- Sublimation — solid → gas (energy absorbed)
- Deposition — gas → solid (energy released)
Think of the three states of matter like the floors of a building. Moving up (gaining energy) takes effort — you need to absorb energy to climb from solid → liquid → gas. Moving down (losing energy) releases energy — like sliding back down.
Energy and Particle Behaviour During Phase Changes
To understand why phase changes need energy, we need to think about what is happening at the particle level.
In a solid, particles are held tightly together in fixed positions by strong intermolecular forces. They vibrate but do not move freely.
In a liquid, particles have enough energy to move past one another, though they are still relatively close together.
In a gas, particles have enough energy to break free almost completely and move rapidly in all directions with large spaces between them.
During a phase change, the energy being added or removed is used to break or form intermolecular bonds — it does NOT increase the kinetic energy of the particles. This is the key reason why the temperature does NOT change during a phase change, even though energy is still being transferred.
Imagine trying to stretch a bungee cord. You can put a lot of energy in — pulling and pulling — without the cord actually moving anywhere new. All your effort goes into stretching the bonds. Phase changes are similar: energy goes into breaking the bonds between particles rather than speeding them up.
10 more sections in this topic
Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.